Yea she'll get together with another guy there. They don't consciously plan this, it just happens. They can always fall back on, "oh I didn't plan it, it just happened, I guess it was meant to be".
Girls cheat all the time. Not intentionally, but it happens. Your best course of action would be to cheat on her, too. Cheat on her now. Don't wait three months. That way when she comes back and says, "baby, I'm sorry I have something to tell you". You can be like, "don't worry about it, I nailed your sister when you were gone. And her best friend".